1. Field of the Invention
The present invention relates to a printing apparatus that creates dots to print an image on a printing medium. More specifically the invention pertains to a printing apparatus that enables a plurality of dots to be formed in each pixel, so as to ensure three or more levels of density expression with regard to each pixel.
2. Description of the Related Art
A variety of printers have been used widely as the output apparatus of the computer to print images in a multi-color, multi-tone expression. One of such printers is an ink jet printer that forms dots with several color inks ejected from a plurality of nozzles provided on a print head and thereby records an image. The ink jet printer generally allows expression of only two levels, that is, the dot on level and the dot off level, with regard to each pixel. The ink jet printer accordingly prints an image after the halftone processing that expresses the various tones of original image data by a distribution of dots.
Multilevel printers that enable tone expression of three or greater levels have been proposed as the technique of ensuring the rich tone expression. Examples of such multilevel printers include a printer that enables at most N dots (where N is an integer of not less than 2) to be created in each pixel, in order to ensure expression of multiple tones. This prior art multilevel printer enables expression of (N+1) levels including creation of no dots, thereby attaining the high quality printing with smooth tone expression. In this multilevel printer, an increase in maximum number N of dots created in each pixel extends the range of possible tone expression. Compared with the printer that varies the quantity of ink or the density of ink, this multilevel printer readily attains the tone expression in a wider range.
In the prior art multilevel printer, the increase in maximum number N of dots created in each pixel lowers the printing speed. Namely it is required to lower the speed of the main scan in the case where N dots are created consecutively in each pixel. It is alternatively required to increase the number of passes of the main scan in the case where N dots are created in each pixel by plural passes of the main scan. In either case, the printing speed is lowered. In the prior art multilevel printer that enables at most N dots to be created in each pixel, the maximum number N of dots created in each pixel is determined by taking into account the conflicting factors, that is, the improved picture quality with smooth tone expression and the sufficient printing speed.
In the prior art multilevel printer, there has been no discussion on the positions of dots formed in each pixel. It is thus possible to further improve the picture quality by taking into account the dot forming positions. This is true not only in the ink jet-type multilevel printers but in any multilevel printers that enable a plurality of dots to be created in each pixel.